Ok, I've asked for some feedback from D-Link.
I can only remember one experience with MAC Cloning and that was at my dads place in KY, he had a Arris modem and I want to say Charter ISP as well. I think. When I visited and installed a wireless router, I had to MAC Clone the router to get the PC and wireless to work.
Things may have changed I presume and thanks for posting what charter said.
I would recommend this if you can and have a local place to buy routers from that has a refund policy, either D-Link or something else, try a different router say for a few days. If this different router doesn't need MAC Cloning then we'll know that there could be a problem with the 605L.
I'll post back when I hear back from D-Link.